I did sail on one back in the day for a week. It was fast, and it still is relatively fast given it's light
displacement, relatively long waterline length, and modern underbody. I also think they are pretty good looking boats, with the lines making it somehow sleek despite the high freeboard.
Can't comment on the build quality over time but people are certainly refitting Columbias to this day. The 50 was considered a well built boat, in general, but the 43 was designed as lighter
displacement.
I would be very careful with respect to condition. These are old boats and you could easily buy yourself a
money pit of epic proportions if you're not careful and don't have experience evaluating older boats and don't know what is involved in terms of time and
money to bring them back. Even if all you're looking for is putz-around boat for weekends with your mates, you don't want to get stuck with some huge
repair down the road just to keep the boat serviceable.
Get a
survey. Be present at the
survey and shadow the
surveyor. Ask lots of questions. If the
surveyor flags something as needing
repair or being past it's
service life, ask for an estimate of what it will cost, all in, to make it right.
As far as speed goes, there is a general calculation:
As a very general rule the maximum speed of any displacement hull--commonly called its hull speed--is governed by a simple formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL).
Being a light displacement
racer cruiser, the 43 is probably going to be at the higher end.
Downwind in 25 knots...if you're sailing conservatively with reefed
sails, you're looking at 7-8 knots surging to 10. Being new to sailing, in 25 knots you
do want to sail conservatively.